U.S. Pat. No. 4,083,709 discloses the production of glass-ceramic articles having integral surface layers exhibiting ferrimagnetic properties wherein the interior portion thereof consists essentially of beta-spodumene solid solution crystals dispersed within a glassy matrix and the surface layer consists essentially of CoFe.sub.2 O.sub.4 and/or NiFe.sub.2 O.sub.4 crystallites dispersed within a glassy matrix. Such articles are prepared by melting a predetermined batch in the Li.sub.2 O-Al.sub.2 O.sub.3 -Fe.sub.2 O.sub.3 -NiO and/or CoO-SiO.sub.2 -TiO.sub.2 composition field, simultaneously cooling the melt and shaping a glass article therefrom, and then exposing the glass article in an oxidizing atmosphere to temperatures between 900.degree.-1300.degree. C. to effect the growth in situ of the desired beta-spodumene solid solution crystals in the body portion of the article and of CoFe.sub.2 O.sub.4 and/or NiFe.sub.2 O.sub.4 crystallites in the integral surface layer.
U.S. Application Ser. No. 946,809, filed concurrently herewith by the present applicant, describes the production of composite glass articles having a glassy interior portion and a thin, integral surface layer containing CoFe.sub.2 O.sub.4, NiFe.sub.2 O.sub.4, and (Co,Ni)Fe.sub.2 O.sub.4 crystallites demonstrating ferrimagnetic properties. Such articles are prepared by melting a proper batch selected from the Li.sub.2 O and/or Na.sub.2 O-FeO-CoO and/or NiO-Al.sub.2 O.sub.3 -SiO.sub.2 composition field, simultaneously cooling the melt and shaping a glass article therefrom, and subsequently exposing the glass article in an oxidizing environment to temperatures between 725.degree.-875.degree. C. to effect the desired in situ growth of CoFe.sub.2 O.sub.4, NiFe.sub.2 O.sub.4, and (Co,Ni)Fe.sub.2 O.sub.4 crystallites in the surface of the article.
U.S. Pat. No. 3,962,514 discusses the manufacture of glass-ceramic articles containing beta-quartz and/or beta-spodumene solid solution as the predominant crystal phase in the interior portion thereof and which have an integral surface layer containing transition metal spinel-type crystals. As illustrative of such spinel-type crystals, the patent reported, among others, NiAl.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, CoFe.sub.2 O.sub.4, and NiFe.sub.2 O.sub.4.
The base glasses for such articles are stated to reside in the Al.sub.2 O.sub.3 -SiO.sub.2 and Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 systems with Li.sub.2 O or MnO.sub.2 as the primary modifying metal oxide. The glasses are nucleated with TiO.sub.2 and/or ZrO.sub.2. The transition metal oxides are present in amounts between 0.1-10% to form the spinel-type crystals.
The precursor glass articles are crystallized in situ to glass-ceramic articles by heating between 800.degree.-1200.degree. C. Thereafter, the integral surface layer containing spinel-type crystals is developed in situ by firing the glass-ceramic article in a reducing environment at 500.degree.-1000.degree. C. In the preferred practice, the glass-ceramic article is treated in an acid solution prior to firing in a reducing atmosphere.
The preparation of glass-ceramic articles had its genesis in U.S. Pat. No. 2,920,971 and reference is hereby made to that patent for further information regarding the physical properties and internal structures of glass-ceramic articles, as well as the mechanism and temperature parameters involved in the nucleating and crystallization of such articles. The glass-ceramic articles of the instant invention, as described hereinafter, are prepared utiizing the general method described and have a microstructure similar to that described in that disclosure.